Secular? Sacred? Metrical structure? Is it (or can it be) broken into two or more parts?

Ursula Vaughan Williams's "A Hymn for St Cecilia" comprises 24 lines of iambic pentameter, broken into three 10 10. 10 10. D stanzas. There are a few settings of this, including mine, posted here in another thread and also published at CPDL.

George Herbert's "Love bade me welcome" comprises 18 lines divided into three stanzas, according to the rhyme & rhythmic scheme, and it has famously been set by Ralph Vaughan Williams in his "Five Mystical Songs" (I've sung this many times in the past), as well as by three other composers whose scores are available at CPDL, including our own Andrew Malton.

The structures of the "Te Deum" and the "Gloria in excelsis" are longer forms, each of a somewhat poetic nature, as well as being sacred, liturgical texts ... and there are plenty of settings of these texts to consider and perhaps emulate.

Without further elucidation as to the nature of the long poem you are considering, it is difficult to offer more suggestions than these without wandering too far afield.

I was recently looking for several of St John of the Cross' poems in Spanish on the youtubes. Every time I thought I'd landed a decent recitation, it was some guy with a guitar.

Another case study is my a cappella "This Advent Moon" - text by Christina Rossetti (first three stanzas of her poem "Advent"). Attached are the score, a live performance MP3, Performance notes, and notes on the structure of the piece. These reveal much of the creative process over three weeks back in the autumn of 2009 when this work was composed. The text consists of 24 lines, iambic, 86 - 2xC.M.D. - C.M. - 86. There are four thematic elements, one of which occurs three times, the second time in strict inversion (see the performance and structure notes for details).